1. Field of the Invention
This invention relates to an apparatus for sensing alignment marks on two objects when the two objects are to be aligned with each other and producing two signals representing the sensing, extracting a necessary signal component from each of the signals, and composing the extracted signals to obtain a desirable signal.
2. Description of the Prior Art
As an example, during the patterning in the manufacturing process of semiconductors, a wafer and a mask must be aligned with high accuracy and recently, this is usually accomplished automatically. In the automatic aligning, photoelectric detecting means are generally used to detect the position of an object. For example, in an apparatus of the prior art, the object is scanned by using a laser light as a light source and the light beams scattered from alignment mark patterns W and M on the wafer and mask shown in FIGS. 1A and 1B of the accompanying drawings are photoelectrically detected by a photodiode. By utilizing the fact that the output signal thereof includes the information on the distance between the patterns W and M, the relative position of the wafer and mask is detected. Such aligning is accomplished by directing the mark patterns W and M detected in the described manner into a positional relation as shown in FIG. 1C. The mark patterns W and M and a pattern constituting an actual element are in a predetermined relation and therefore, if the mark patterns W and M are directed into a predetermined positional relation, the actual element patterns on the wafer and mask are properly aligned. If the positional relation between the mark patterns W and M is detected, the difference between that positional relation and the predetermined relation may be examined and a driving mechanism may be operated so that this difference becomes null. The aligning of the wafer and mask requires all degrees of two-dimensional freedom to be controlled and is usually accomplished by observing a plurality of locations on the wafer and mask.
The alignment marks depicted in FIGS. 1A and 1B are known from U.S. Pat. No. 4,167,677 of the present assignee.
On the other hand, when the coherent lights coming back from at set of marks on the wafer and mask are detected by a single photoelectric detector, the two lights interfere with each other to provide an unstable signal and this has led to the disadvantage that automatic aligning becomes impossible or the accuracy of aligning deteriorates.
In a specific example shown in FIG. 5 of U.S. Pat. No. 4,251,129 of the present assignee, the light signals from the wafer and mask are detected by different photoelectric detectors, whereby a stable signal is obtained. However, even if great improvements have been made by this apparatus, undesirable signals concerning the mark of another object to be detected can remain in the signals of the detectors.